Feature Article: Mangroves In Reef Aquaria

Growing mangroves enables one to decorate a tank to look like a fringing reef that surrounds an island, as seen from the ocean. In fact, this is what I tried to do with my 1500 gallon tank with the help of 40 mangroves.

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Several years ago reef enthusiasts started to take a fancy to mangroves. To my knowledge this was something "made in the USA," because Julian Sprung was the one who got it started, which was not surprising for someone who lives in Miami Beach, Florida. He planted a mangrove in his 15 gallon reef tank, and to everyone's surprise - maybe even to his own surprise - it started to grow wonderfully and still does to this day. This was copied by many reefkeepers, and after Julian had published an article about mangroves in a German aquarium magazine, even German reefkeepers got keen to plant those little trees into their tank.

Growing mangroves enables one to decorate a tank to look like a fringing reef that surrounds an island, as seen from the ocean. In fact, this is what I tried to do with my 1500 gallon tank with the help of 40 mangroves. A viewer that sits in front of the tank faces the "wall" of a "fringing reef," and if he looks over the water surface, he can see a "beach area" behind it, planted with mangroves. When standing behind the tank, one feels like a giant looking at a little mangrove forest over the beach area into the coral reef. Sometimes you can even see corals and other sessile inverts growing on the submerged parts of the mangrove's trunks. I remember a Pavona venosa fully surrounding one of my mangroves, covering the complete submerged trunk - a little unusual because Pavona cannot be considered a typical coral genus growing in the mangrove area. But, in fact, those mangroves growing near the lagoonal area where their roots are permanently submerged even during
ebb tide often have lots of sponges, mussels and even corals growing on them. Pocillopora damicornis grows in this area, and occasionally we can also find corals typical for a lagoon. If we plant mangroves in our aquaria and we want to create a coral community that has some similarity to the low and submerged parts of a natural mangrove zone, we can use Catalaphyllia, Goniopora, Pachyseris, Leptoseris, Herpolitha, Euphyllia, and many others. Even small polyped scleractinians like Porites, Montipora, Pavona, Pocillopora and even some Acropora species can occasionally be found there, aside from zoanthid polyps, disc anemones and soft corals like Sarcophyton or Sinularia.

What Is The Advantage Of Having Mangroves In A Reef Tank?

Mangroves take the nutrients necessary for their growth from the aquarium water. This means that we have a means of exporting phosphates and nitrates. Macro algae do the same, but they easily set those nutrients free when they are eaten by fish or die and dissolve. With mangroves this is different, at least if the aquarist succeeds in preventing the mangrove leaves from falling into the water and dissolving there. While many mangrove species export excess salt by depositing it on the surface of their leaves for the rain to wash away, some mangrove species deposit excess salt inside of their oldest leaves, which then will turn yellow and drop down. This is a natural process, but in the reef tank we just have to make sure that the leaves will not dissolve in the aquarium water and release nutrients back into the aquarium water.

But, on the other hand, regarding the nutrient export capacity of some mangrove plants living on the upper zone of our reef tank, we should not expect miracles. They are slow-growing plants, and their nutrient uptake is limited. To say it clearly: if we have the problem of exporting phosphates and/or nitrates from our tank, due to over-feeding, insufficient foam fractionation, etc, we will certainly not be able to solve it by planting mangroves. Having mangroves in the tank just helps to make the man-made biotope a little more natural, in function and appearance. If we try to create something we call a "mini reef," we should take every opportunity to employ natural mechanisms. Even though their functional contribution to the system is relatively small, it makes our "mini reef" a bit more natural.

For Which Types Of Reef Tanks Are Mangroves Suitable?

Of course, a tank where we want to plant mangroves needs to be open on the top, so the plants have space to grow. Theoretically, we can also cut an opening into the light hood of a closed tank, but from an aesthetically point of view this can hardly be satisfying, at least if one tries to copy a natural beach area. The most fascinating thing with mangroves in a reef tank is - at least to my opinion - the view from the top in front of the tank on the little "coral reef" that extends from the "deep ocean" to the "shallow lagoonal area," ending in the mangrove-populated "coastal zone." With a light hood this does not work.

Another type of tank for mangroves is a special mangrove-aquarium. This can even be done with a filter tank and is very simple. Julian Sprung has a tank of this type connected to his "famous" little reef tank. All you have to do is to take a few mangroves that root on some porous lime rocks, place it in the center of this tank and surround it with some corals typical of the lagoonal zone. A weak water current is sufficient for the mangroves (but obviously you should also ask the corals for their opinion). This mangrove tank can be connected to your main reef tank, for example by placing it a little higher and pumping a small amount of water from the reef tank into the mangrove tank, so it will flow back into the reef tank via gravity. With the help of a timer you can even add a second pump which will pump at certain intervals and transport more water into the mangrove tank, resulting in a higher water level. By doing so you will imitate the natural high and ebb tides. If you create
the decoration of your mangrove tank in a way that lets some areas fall dry during ebb tide, you can even add animal life typical for this mangrove zone, a fascinating thing.

Mangrove Propagules Or Mangrove Seeds?

Mangrove species have developed different strategies for propagating. Those species growing on the more elevated areas near the land which falls dry during ebb tide develop propagules instead of seeds, e. g. Bruguiera- and Rhizophora-species (mostly R. mangle ). A propagule is a sprouting young plant that looks a bit like a candle. When it falls down from the mangrove tree at ebb tide, it can bore itself deeply into the muddy ground. This helps to prevent it from being washed away when the sea water returns during high tide. One must know that once those mangrove propagules have been washed away from the shore area reaching the open sea, the chances of their survival by finding another shoreline of an island is very limited and the long term survival of those propagules would be very questionable, though they can survive even three months floating around. Other mangrove species inhabiting other areas of the shoreline develop other strategies of
propagation and form seeds, mostly in the size of a hazelnut.

The propagules as well as the seeds can be used in a reef tank. The seeds grow into a plant with a thin stem that can perfectly adjust its growth form to the light conditions on top of the aquarium, but they seem to be a bit more sensitive if the halide lamps emit too much heat. Propagules seem to be hardier, but they are also larger and in a very small reef tank they might look unsuitably large, while a mangrove plant that has grown from a seed may have a more natural tree-shape, giving the scene a more natural appearance. A few years ago I was able to get mangrove propagules and seeds from Florida and the Philippines. In my experience it was much more difficult to keep the seeds in place. Even when it was securely fixed between the rocks, some fish came nibbling around on the seeds. In contrast to this, propagules can easily be put between some lime rocks. In my case, the propagules and seeds grew nicely during the first few weeks, but then about one half of the mangroves grown
from seeds died. This may have been caused by planting them too near to the 1000 watt halide lamps, which emit a lot of heat. All in all, the survival rate has been much better among the mangroves grown from propagules, though today I still have some mangroves grown from seeds.

Planting Mangroves In A Tank Is Simple

Under natural conditions, mangroves do not only root in mud, but also in lime rock. Consequently, we can offer them some porous lime rocks placed in the upper area of the tank. The simplest way is if you push the propagules between two or three porous rocks, allowing their roots to grow into the pores. If the propagule already has developed fine roots when you get it, you can also lay those roots around a porous lime rock and carefully fix it with a rubber band, waiting for the roots to hold tightly to the rock This way it is easy to change the plant's location at a later time, though this should be avoided as far as possible because the plants strongly adjust to their environment, especially to the illumination.

But a single mangrove has little similarity to a mangrove forest. If you want to have a "real mangrove forest," you may need to employ a different way of fixing the plant in the water. For this purpose, I have embedded the propagules into rock wool and placed the whole thing in a small grid basket formed of plastic commonly used for freshwater aquarium plants. The roots grow through the rock wool and the holes in the basket, holding the plant tight. This basket makes it easy to connect several mangroves to each other by using nylon cable ties. That way I have created groups of mangroves that stabilize each other in their position.

If you want to create a "beach zone" with fine white sand and mangroves on top of your tank, you can go one step further and connect those plastic baskets to a perforated plastic sheet (PVC or acrylic). This sheet with mangroves then gets placed in the upper area of your tank, where you can cover it with coral sand, coarse grain size on the bottom of the sheet, and covered with fine white coral sand.

Illuminating Mangroves

The simplest illumination for the mangroves is the light emitted at the side of a halide lamp. But you need to make sure that the plants don't grow directly under the lamps because of the strong heat emitted there. Also, the plants would shade corals when growing directly under the lamps. The stronger your halide lamp, the greater the distance you need to plant the mangrove from it. But, if necessary, you can cut the plant in shape at a later time when it grows branches too near to the lamp. Also the color temperature of the lamps is of importance for the mangroves. The best light for mangroves is, of course, a daylight lamp at 6,000 Kelvin, since they are land plants. With a lamp of 10,000 Kelvin it may also be possible to grow mangroves, but a 20,000 Kelvin lamp will probably make it harder to satisfy the physiological needs of mangrove plants (though I have not tried it).

Mangroves And Daylight

If the aquarium is placed under a window, we can also use the natural daylight to grow mangroves. In this case, we can even illuminate the tank with fluorescent tube lamps, even with a closed lamp hood having a hole for the mangrove. An alternative to the natural daylight or the halide lamp would be a special plant lamp hanging on top of the mangrove. That helps placing the lamp a greater distance from the mangrove, and also permits putting it right on top of the plant, resulting in a more natural looking growth form.

Maintaining Mangrove Plants

Mangrove plants don't need much care. The most frequently traded mangroves are Rhizophora mangle, which exports salt by producing a thin layer of salt crystals on top of their leaves. This should be washed away daily - or at least two to three times per week - by spraying fresh water on top of the mangroves. We use distilled water for this purpose. Be very careful when spraying water on to the mangroves on top of the aquarium if there are lamps and electrical outlets! That is about all you have to do other than cutting some branches occasionally or even the growth tip of the plant if it comes too near to the lamp.

A view on the mangrove zone of the authors' 1500 gallon reef tank with a "floating coral nursery". Photo: Daniel Knop

Occasional cleaning of the mangroves from salt is helpful. Photo: Daniel Knop

When mangroves propagules are placed in plastic grid baskets it is easy to attach it to a perforated sheet with the help of nylon cable ties. Diagram: Daniel Knop

It is best to buy mangrove propagules when they have a protection for the roots and some green, healthy leafes. Photo: Daniel Knop

Zonation Of A Natural Mangrove Forest

The zonation of a natural mangrove forest starts with very salt-resistenrt species of the genus Sonneratia (1) on the sea side that are capable of tolerating a permanent submersion of their roots. Moving more near to the shore at the upper end of the intertidal zone we mostly find Rhizophora species (3), which is the most commonly offered mangrove species in the aquarium trade. Here they are submersed only for a limited period of time. Species of the genus Bruguiera (4) have more sensitive roots that are not capable of living unter the rough conditions we find in the intertidal zone. Therefore they mostly grow over the intertidal zone in the mud collecting here.

Tipps For Buying And Keeping Mangroves:

Prefer propagules instead of seeds, if available

Try to get the species Rhizophora mangle, because it is very hardy and adjustable

Buy a propagule with some roots and some leafes

Make sure that the roots are not hurt. It is best if the roots have grown into rock wool, covered by a pot.

Chose a propagule with healthy green leafes. yellow or dried leafes can be something natural, but as well it can be a sign of transport damage.

Some mangrove species - especially Bruguiera sp., react very sensitive to infections with molds fungi, which sometimes develop in the humid conditions near an open aquarium. Try to avoid the development of molds on the walls and ceiling if you want to keep mangroves.

Web Links:

There are some excellent hyperlinks to mangrove biology available on the web, the first (especially) and the third are excellent resources. The fourth gives the three species of mangrove found in South Eastern US. Craig Bingman, Science editor.